A) Field of the Invention
The present invention relates to a semiconductor device and its manufacture method, and more particularly to a compound semiconductor device having a channel (electron transfer) layer made of GaN group compound semiconductor and its manufacture method.
B) Description of the Related Art
GaN field effect transistors have been developed which use as a channel layer a GaN layer in contact with an AlGaN/GaN hetero junction. GaN has a wide band gap, a high breakdown electric field strength (intensity) and a high saturated electron velocity and is quite promising as the material of devices of high voltage operation and high output power.
A power device in a cellular phone base station is required to operate at a high voltage of 40 V or higher so that GaN field effect transistors present bright prospects. High voltage operation requires a high gate breakdown voltage (a high reverse gate voltage at a predetermined gate leak current, and a high gate-drain voltage at a predetermined gate-drain leak current).
FIGS. 8A and 8B are a cross sectional view and a graph showing the structure and characteristics of a conventional GaN field effect transistor (FET).
Referring to FIG. 8A, on a substrate made of sapphire or SiC, a buffer layer of GaN or AlN is formed when necessary, to form a work substrate 1. On this work substrate 1, a GaN channel (electron transfer) layer 2 is formed. An AlGaN electron supply layer 3 is formed on the GaN channel layer 2. The electron supply layer 3 has an n-type conductivity doped with, for example, Si and can supply electrons to the channel layer 2.
On the electron supply layer 3, a gate electrode 5, a source electrode 6 and a drain electrode 7 are formed, and the surface of the electron supply layer 3 is covered with a passivation film 4.
FIG. 8B shows the band structure of a semiconductor layer under the gate electrode of the semiconductor device shown in FIG. 8A. The abscissa represents a film thickness form the semiconductor surface in the unit of nm, and the ordinate represents the energy at the bottom of the conduction band in the unit of eV. It is known that GaN group semiconductor has large piezo polarization effects and large spontaneous polarization effects. These polarization effects raise the potential energy of the conduction band, from the interface between the GaN channel layer 2 and AlGaN electron supply layer 3 toward the surface of the AlGaN electron supply layer 3. In the GaN channel layer 2 at the interface with the AlGaN electron supply layer 3, two-dimensional electron gas (2DEG) is accumulated.
A tunneling current I through a potential barrier of length L has the following relation.I∝exp(−C×L)
The tunneling current increases as the potential barrier length L becomes short. As shown in FIG. 8B, there is a region where the potential energy of the conduction band in the AlGaN layer increases steeply and the potential barrier length becomes shorter. Through this region, electrons supplied from the surface (gate electrode) are likely to be tunnelled. A two-terminal breakdown voltage is about several ten V which is insufficient for high voltage operation.
FIGS. 8C and 8D are a cross sectional view and a graph showing the structure and characteristics of an improved GaN-FET device.
As shown in FIG. 8C, as compared to the structure shown in FIG. 8A, an n-type GaN cap layer 8 is disposed between the electron supply layer 3 and gate electrode 5.
FIG. 8D shows the potential energy distribution of a conduction band bottom in the structure shown in FIG. 8C. The abscissa represents a film thickness from the substrate surface in the unit of nm and the ordinate represents the energy of the conduction band bottom in the unit of eV. Negative charges are accumulated at the interface between the n-type AlGaN electron supply layer 3 and n-type GaN cap layer 8 so that the potential energy increases from the surface of the n-type GaN cap layer 8 toward the AlGaN electron supply layer 3. Therefore, the peak potential energy of the conduction band at the interface between the n-type AlGaN electron supply layer 3 and n-type GaN cap layer 8 becomes high and the peak position moves from the substrate surface to a deeper position.
The band structure is changed in this manner so that a tunneling current from the substrate surface can be suppressed. It is possible to set a two-terminal gate breakdown voltage to 150 V or higher and a three-terminal gate breakdown voltage to 50 V or higher (for example, refer to Japanese Patent Laid-open Publication No. 2002-359256 which is incorporated herein by reference).
Japanese Patent Laid-open Publication No. 2001-230407 proposes a field effect transistor whose GaN cap layer under the source and drain electrodes is removed.
Japanese Patent Laid-open Publication No. 2002-16087 proposes a field effect transistor whose source and drain electrodes are formed on an AlGaN electron supply layer and a gate electrode is formed on an InGaN Schottky-contact-forming layer.